Short/medium chain fatty acids (S/MCFAs, e.g. C6-C12) are precursors of many industrial chemicals and biofuels. The main source of S/MCFA is coming from refining of vegetable oil and fossil oil, but world availability is limited for these compounds while consumption is increasing. Therefore, it is of special interest to find new source for these products and in the same time to produce these kinds of fatty acids in a sustainable way by transformation of biomass-based materials by microbes.
The natural process of microbial fatty acid synthesis proceeds via a stepwise addition of two carbon units onto a growing acyl chain bound to acyl carrier protein (ACP). The process begins as a condensation of acetyl-ACP and malonyl-ACP into acetoacetyl-ACP liberating CO2 which drives the reaction forward. The second step involves reduction of acetoacetyl-ACP to D-3-hydroxybutyryl-ACP using NADPH. Following a dehydration to crotonyl-ACP and another reduction using NADPH, butyryl-ACP is formed. The chain elongation typically continues with further addition of malonyl-ACP until an acyl chain of certain length is formed, which is then hydrolyzed by a thioesterase into a free fatty acid.
Recently, most efforts for chain length control were focused on the fatty acid synthesis or reversal of β-oxidation, and the targets for limiting acyl chain elongation are the enzymes responsible for condensation (ketoacyl-ACP synthase and thiolase) or product release (acyl-ACP/CoA thioesterase).
The reversal of the β-oxidation was first established in E. coli, and individual functional parts of this pathway were identified. However, this pathway prefers to produce short chain carboxylic acid (butyric acid). Further engineering involves using another thiolase (such as BktB) able to condense acetyl-CoA with C4-C8 acyl-CoA intermediates to produce medium chain fatty acids. The titers in E. coli were more than 1 g/L (C6-C10). Recently, after testing more than 40 enzymes, the reversed β-oxidation was realized in yeast, although the capacity is much lower than that in E. coli. 
In prokaryotes, the dissociated type II fatty acid synthases are used. The end product of fatty acid synthesis is acyl-ACP. A common strategy for producing S/MCFA is expression of short chain acyl-ACP thioesterases, which release S/MCFA from ACP thioester. The short/medium chain acyl-ACPs, the substrates of thioesterases, are also incorporated into long chain fatty acid synthesis. So a cerulenin resistant β-ketoacyl-ACP synthase mutant (FabF*) with lower affinities to medium/long chain acyl-ACP (C8-C14) was used to block the incorporation of short/medium chain acyl-ACPs. Combined strategies used in E. coli resulted in titers of 118 mg/L C8 fatty acid.
From the above it is clear that various strategies are investigated and employed in order to produce fatty acids, such as S/MCFA, in particular microbial production of fatty acids. There however remains a need in the art for new and improved methods for fatty acid production, such as S/MCFA. The present invention provides hereto a solution.